A novel bedtime pulsatile-release caffeine formula ameliorates sleep inertia symptoms immediately upon awakening

Sleep inertia is a disabling state of grogginess and impaired vigilance immediately upon awakening. The adenosine receptor antagonist, caffeine, is widely used to reduce sleep inertia symptoms, yet the initial, most severe impairments are hardly alleviated by post-awakening caffeine intake. To ameliorate this disabling state more potently, we developed an innovative, delayed, pulsatile-release caffeine formulation targeting an efficacious dose briefly before planned awakening. We comprehensively tested this formulation in two separate studies. First, we established the in vivo caffeine release profile in 10 young men. Subsequently, we investigated in placebo-controlled, double-blind, cross-over fashion the formulation’s ability to improve sleep inertia in 22 sleep-restricted volunteers. Following oral administration of 160 mg caffeine at 22:30, we kept volunteers awake until 03:00, to increase sleep inertia symptoms upon scheduled awakening at 07:00. Immediately upon awakening, we quantified subjective state, psychomotor vigilance, cognitive performance, and followed the evolution of the cortisol awakening response. We also recorded standard polysomnography during nocturnal sleep and a 1-h nap opportunity at 08:00. Compared to placebo, the engineered caffeine formula accelerated the reaction time on the psychomotor vigilance task, increased positive and reduced negative affect scores, improved sleep inertia ratings, prolonged the cortisol awakening response, and delayed nap sleep latency one hour after scheduled awakening. Based on these findings, we conclude that this novel, pulsatile-release caffeine formulation facilitates the sleep-to-wake transition in sleep-restricted healthy adults. We propose that individuals suffering from disabling sleep inertia may benefit from this innovative approach. Trials registration: NCT04975360.

behavioral and cognitive deficits associated with sleep inertia 13 . The proportion of deep sleep and the odds of awakening from N3 sleep is increased when a sleep opportunity is shorted such as in sleep restriction. Accordingly, abrupt awakening during sleep restriction enhances sleep inertia symptoms 4 .
The neuromodulator adenosine is a key regulator of deep sleep 14 and adenosinergic neuromodulation may play an essential role in the manifestation of sleep inertia 15 . Consistent with this view, the adenosine receptor antagonist, caffeine, is widely used to counteract sleep inertia. Besides attenuating sleepiness and deficits in vigilance 16,17 , caffeine augments cardiovascular and respiratory functions 18 and promotes the release of cortisol, a key hormone of the hypothalamic-pituitary-adrenal (HPA) axis 19 . The HPA axis regulates several psychovegetative aspects of the wake-up process, including the cortisol awakening response (CAR). The CAR reflects HPA axis function 20,21 and may be associated with the propensity of sleep inertia 22 .
Because the impairments are most severe immediately upon awakening, proactive strategies aiming for optimal sleep length and timing have been recommended to minimize sleep inertia symptoms 6 . Nevertheless, it is not always possible to plan and obtain sleep of sufficient length and quality and at the optimal time of day. On the other hand, currently there exists no convincing evidence that reactive countermeasures to sleep intertia, i.e. strategies implemented upon wake-up, are sufficiently effective 6 . Although caffeine is the best available option, coffee takes 20-30 min to have an alerting effect 23 . The bioavailability of reactive oral caffeine intake prevents effective amelioration of sleep inertia symptoms for at least 12-18 min upon waking 17 , unless it is taken as a proactive countermeasure prior to a short sleep or nap bout 16,24 . When taken before sleep, however, caffeine can delay sleep onset, reduce total sleep time and attenuate the amount of deep slow wave sleep when a pharmacologically active concentration is present in the organism during sleep 24,25 .
The time-controlled administration of pharmaceuticals in accordance with the sleep-wake cycle provides an essential pillar in the emerging concept of chronopharmacology and chronotherapeutics 26,27 . We aimed at developing a chronotherapeutic caffeine formulation that ameliorates impaired subjective state, vigilance and performance immediately upon awakening without disturbing the quality of the preceeding sleep episode. For this purpose, we invented a delayed, pulsatile-release caffeine delivery system targeted to reach an efficacious plasma concentration approximately 7 h after intake. When ingested at habitual bedtime, we hypothesized that this formula would improve vigilance and mood, elevate the CAR, and reduce sleep propensity on the subsequent morning after wake-up from nocturnal sleep. We tested these hypotheses in two separate studies. First, we examined the in vivo release properties of the engineered caffeine formula throughout a nocturnal sleep episode. Then, we comprehensively investigated in randomized, double-blind, cross-over, placebo-controlled manner its effects on behavioral, emotional, neurocognitive and physiological symptoms of sleep inertia in sleep-restricted healthy young men.

Methods
Participants and permission. A total of 32 healthy young men (mean age: 25.6 ± 3.7 years) participated in the two studies (in vivo validation study: n = 10; pharmacodynamic study: n = 22), whereof 5 subjects participated in both experiments. The following criteria were required for inclusion: (i) male sex in order to avoid the potential impact of menstrual cycle on sleep physiology or HPA axis activity, (ii) age within the range of 18-34 years, (iii) a body-mass-index below 25, (iv) an Epworth Sleepiness Score (ESS) below 10, (v) habitual sleep onset latency below 20 min, (vi) regular sleep-wake rhythm with bedtime between 10 pm and 1 am, (vii) absence of any somatic or psychiatric disorders, (viii) no acute or chronic medication intake, (ix) non-smoking, (x) no history of drug abuse (lifetime use > 5 occasions, with exception of occasional cannabis use), and (xi) caffeine consumption of less than 4 units per day (coffee, tea, chocolate, cola, energy drinks).
The participants were instructed to abstain from illicit drugs and caffeine during the entire study, starting two weeks prior to the first experimental night until the end of the study (the day after the second experimental night). No alcohol was allowed 24 h before the expermental nights. The minimal wash out period before the experimental nights was 7 days. Participants were also instructed to keep an individual regular sleep-wake rhythm (23:00-07:00 or 22:00-06:00 depending on the volunteers' habitual bedtime) during the entire study, starting 2 weeks prior to the experimental night. All included participants chose to keep either a 22:00-06:00 rhythm or a 23:00-07:00 rhythm. To ensure adherence to the regular sleep-wake pattern, participants were instructed to wear a rest-activity monitor on the non-dominant arm and to keep a sleep-wake diary.
The studies were approved by the Cantonal Ethics Committee of the Canton of Zurich (BASEC: 2018-00533) and registered on ClinicalTrials.gov (Identifier: NCT04975360). All participants provided written informed consent according to the declaration of Helsinki. Study drug. The caffeine pulsatile-release formulation was manufactured using a drug layering process 28 .
The details of the engineering and manufacturing processes will be reported elsewhere. In brief, caffeine and the excipients were dispersed in the coating media and then sprayed onto inert microcrystalline cellulose spheres using a fluid bed through a Wurster tube with continuous inlet air that dries the liquid in the dispersion, to obtain various layers consisting of caffeine and release-controlling polymers. The applied release-controlling polymeric system was based on methacrylate copolymers, which control the release of caffeine in both a pH-dependent and pH-independent manner 29,30 . Thereby, the release mechanism of the polymeric system was mainly driven by the swellability and permeability of the copolymers 31 . The final micropellets were then encapsulated into hydroxypropyl-methylcellulose capsules.
To evaluate the in vitro dissolution profiles of the manufactured formula, different prototypes were tested by means of state-of-the-art dissolution assays, mimicking gastrointestinal conditions 32,33 . Development and in vitro testing of the caffeine pulsatile-release formulation and placebos was conducted at Elixir Pharmaceutical

In vivo validation study.
In a first open-label evaluation study of the engineered delivery system, the in vivo caffeine release profile was determined in 10 fasted (no food or beverage consumption 2 h before drug administration) male individuals. After oral intake at 22:30, study participants were allowed to sleep from 23:00 to 07:00, while blood was continuously sampled. Samples were collected from the left antecubital vein at baseline (22:00), and 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5, 9.5, 10.5, 13.5 and 17.5 h after drug administration. During the sleep episode in the soundproof and climatized bedrooms of the sleep laboratory, the venous catheter was connected to a blood-collection setup in an adjacent room (Heidelberger plastic tube extensions through the wall). Thus, blood samples (4 ml, BD Vacutainer EDTA) were collected without disturbing the sleeping study participants. The intravenous line was kept patent with a slow drip (10 ml/h) of heparinized saline (1000 IU heparin in 0.9 g NaCl/dl; HEPARIN Bichsel; Bichsel AG, 3800 Unterseen, Switzerland). Blood samples were immediately centrifuged for 10 min at 2000 RCF and plasma samples were immediately stored on ice until final storage at − 80 °C.
Pharmacodynamic study. In the pharmacodynamic study, the pulsatile-release caffeine formulation (which was validated in vivo as described above) or a placebo (matched in appearance) were administered 8.5 h before the scheduled wake-up time to the fasted participants (no food or beverage consumption 2 h before drug administration). To exacerbate sleep inertia symptoms and avoid neuropsychological ceiling/floor effects, which are frequent in intervention studies with highly functioning healthy volunteers, all participants were sleep restricted. More specifically, they were kept awake until 02:00 (participants adhering to a 22:00-06:00 rhythm) or 03:00 (23:00-07:00 rhythm), then given a 4-h sleep opportunity, and awoken at 06:00 (22:00-06:00 rhythm) or 07:00 (23:00-07:00 rhythm). At 3.5 h post-administration, all volunteers received a standardized, light meal. Blood was continuously sampled upon drug administration and the caffeine release from the formulation was monitored as described above. Upon awakening, the effects of the formulation on neurobehavioral, emotional, cognitive, and endocrinological markers of sleep inertia were assessed. Additionally, physiological sleep tendency was investigated by determining the sleep characteristics of a 1-h nap opportunity starting 1 h postawakening. To simplify descriptions and data presentations, we will only refer to the 23:00-07:00 rhythm with respect to the time-points of the tasks, because only a small minority of the participants followed the 22:00-06:00 rhythm.
The pharmacodynamic study followed a randomized, double-blind, placebo-controlled, crossover design with a wash-out period of at least 1 week between the caffeine and placebo conditions. The details of both study designs are illustrated in Fig. 1.  In both studies, sleep was continuously recorded by polysomnography. In the pharmacodynamic study, participants were kept awake until 3:00. Immediately after awakening from a restricted 4-h nocturnal sleep episode (at 07:00), volunteers performed a 1-h testing battery (referred to as "Testing") to quantify behavioral, cognitive, emotional and physiological markers of sleep inertia. At 8:00, the participants were given a 1-h nap opportunity, while the latency to fall asleep and the sleep profile were recorded with polysomnography. Sleep inertia assessment, CAR measurements and physiological sleepiness testing were only performed in the pharmacodynamic study. www.nature.com/scientificreports/ centrifuged at 10,000 rpm for 5 min. A volume of 350 µl of the supernatant was transferred into an auto-sampler vial and evaporated to dryness under a gentle stream of nitrogen. For reconstitution, 250 µl of an eluent-mixture (95:5, v/v) was added. Quality control (QC) samples and calibrators (Cal) were prepared with the same sample preparation, replacing the 50 µl of MeOH with the Cal or QC solutions. The plasma samples were analyzed on an ultra-high performance liquid chromatography (UHPLC) system (Thermo Fisher, San Jose, CA, USA) coupled to a linear ion trap quadrupole mass spectrometer 5500 (Sciex, Darmstadt, Germany). The mobile phases of the UHPLC consisted of water (eluent A) and a mixture (70:30 v/v) of MeOH and ACN (eluent B), both containing 0.1% of formic acid (v/v). A volume of 5 µl of the prepared samples was used for quantification. Using a Kinetex Biphenyl column (50 × 2.1 mm, 1.7 µm) (Phenomenex, Aschaffenburg, Germany), the flow rate was set to 0.45 ml/min with the following gradient: start conditions 95% of eluent A, decreasing to 80% in 3 min followed by a quick decrease to 2% within 0.5 min. These conditions were held for 1 min and switched to the starting conditions for re-equilibration for 1 min. The mass spectrometer was operated in positive electrospray ionization mode with scheduled multiple reaction monitoring. Three MRM transitions were used for both analytes. For quantification, the peak area of the analytes was further integrated and divided by the peak area of the IS. Cal samples were fitted with a least-squares fit and weighted by 1/x. The limit of quantification of caffeine was 1.2 µM.

Modified sleep inertia questionnaire (SIQ).
We modified the SIQ 37 to assess volunteers' subjective experience of the awakening process. The original version of the SIQ represents a trait inventory, in which participants are instructed to rate the quality of their awakening process during the last week, namely on physiological, emotional, cognitive and behavioral levels. Thereby, the inventory instruction reads as follows: "On a typical morning in the past week, after you woke up, to what extent did you, for example, have problems to get out of bed" (possible ratings: 1 = not at all, 2 = a little, 3 = somewhat, 4 = often, 5 = all the time). For the present study, we rephrased the inventory's instruction to gain state information of the wake-up process of the experimental morning (rather than trait information of the last week), to analyze the acute effects of our pulsatile-release formula. The instruction was rephrased as follows: "How strong did you feel the following aspects after you woke up this morning compared to a normal morning last week: for example, have problems to get out of bed" (possible ratings: − 3 = extremely less, − 2 = much less, − 1 = a little bit less, 0 = same, 1 = a little bit more, 2 = much more, 3 = extremely more). For our purpose, the modified version of the SIQ was renamed to Acute Sleep Inertia Questionnaire (ASIQ) and was administered at 07:45.
N-back task. At 7:47 the n-back task was executed in 1-, 2-and 3-back versions 38 . Over a period of 7 min, a random series of letters were displayed and subjects were instructed to press a key when the currently displayed letter corresponded to the previous (1-back), the penultimate (2-back) or the antepenultimate (3-back) letter, respectively. The reaction times and the number of correct and incorrect answers were assessed.
d2-task. Finally, the d2-task, a neuropsychological measure of selective and sustained attention and visual scanning speed 39 , was administered at 07:55. In this task, participants were instructed to cross out any letter "d" with two marks above it or below it in any order. The surrounding distractors were either a "p" with two marks or a "d" with one or three marks. For each line (14 lines in total), subjects were given 20 s to mark all "d" with two marks and then instructed to proceed to the next line. The number of correct and incorrectly crossed characters were determined. All questionnaires as well as the n-back and d2 tasks were administered as paper-pencil versions.
Cortisol awakening response (CAR). Cortisone-D 7 was purchased from Sigma Aldrich (Buchs, Switzerland) and 13  www.nature.com/scientificreports/ to chew the swab for 60 s and then return it into the Salivette ® tube (Sarstedt, Germany). After sampling, tubes were immediately stored on ice until final storage at − 80 °C. For cortisol detection, tubes were defrosted and centrifuged for 5 min at 5000 rpm to yield clear saliva in the conical tube. Two subjects had to be excluded, as the amount of saliva yielded from the swabs was insufficient. Then, the swab was removed and the yielded saliva was spiked with 50 μl IS (0.1 ng/μl Cortison-d 7 ) for further analysis. A fully automated supported liquid extraction (SLE) was carried out by transferring 265 μl saliva into a column rack (24 × 6 ml) from Biotage ® Extrahera (Biotage, Uppsala, Sweden) and adding 300 μl water to the sample. After mixing the extracts were automatically loaded onto Isolute SLE + columns and allowed to absorb for 5 min. Analytes were then eluted two times with 1.5 ml ethyl acetate with a waiting time of 5 min in-between. The extracts were dried in a Turbovap ® (Biotage, Uppsala, Sweden) at 35 °C. The dry residues were resuspended using 150 μl methanol and 350 μl ammonium formate (5 mM) solution, which was used for liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis following a recently published method using 13 C 3 -labeled cortisol as surrogate analyte for calibration 40 1-15). For all applied models, normal Q-Q plots were applied, demonstrating normality of the residuals. Moreover, the assumption of homoscedasticity and linearity was verified using a Tukey-Anscombe plot (residuals vs. fitted). Post-hoc testing was carried out using the 'R' package emmeans (Version 1.2.1). The p values of the post-hoc tests were corrected for multiple comparison using Benjamini-Hochberg correction of the false discovery rate 47 . If not noted otherwise, only significant effects and differences are reported.

Results
Caffeine release profiles. The in vivo validation study during sleep revealed a pulsatile-release profile of the administered formulation; c max (maximal plasma concentration) was reached after 10.5 h (Fig. 2A). The caffeine curve followed a sustained-release profile, and efficacious plasma levels (> 5 μM) were attained after 7 h 48,49 .
Unexpectedly, in the pharmacodynamic study, the caffeine release profile distinctly differed from that in the in vivo validation study. A sustained-release of caffeine started after 3.5 h and efficacious plasma caffeine levels were already attained 5 h post-administration (Fig. 2B). The premature burst was most likely triggered by gastric movements due to a standardized meal that was served to the study participants 3.5 h post-administration in the pharmacodynamic study. This meal was absent in the in vivo validation study.  (Fig. 3). On the other hand, the number of lapses remained unaffected (F = 1.04; p = 0.359).  118), such that the engineered caffeine formulation increased positive ratings (p < 0.01) and tended to reduce negative ratings (p = 0.067) when compared to placebo (Fig. 4).

Acute sleep inertia questionnaire (ASIQ).
The statistical analyses revealed a significant condition effect (F = 31.21; p < 0.001; η 2 = 0.175), such that the engineered caffeine-release formula reduced the ratings on all subscales of the ASIQ (behavioral, cognitive, emotional; physiological; Fig. 4). Remarkably, several individuals reported less problems to rise from bed on the experimental mornings compared to a normal morning in the week preceding the experiment (indicated as negative values in Fig. 4). This notion was particularly true during the caffeine condition (n = number of subjects with negative values: behavioral n = 9, cognitive n = 12, emotional n = 9, and physiological n = 10) and to a minor degree also during the placebo condition (behavioral n = 5, cognitive n = 7, emotional n = 5, and physiological n = 2).
N-back and d2 tasks. The statistical analyses of the n-back working memory (F = 0.43; p > 0.05) and the d2 sustained-attention tasks performance (F = 0.29; p > 0.05; η 2 = 0.001) revealed no significant condition effects (data not shown).
Cortisol awakening response (CAR). The statistical analyses of the salivary cortisol levels revealed no significant main effect (p > 0.05). Nevertheless, post hoc testing revealed significantly increased cortisol levels at 08:00 (60 min post-awakening) in the caffeine condition when compared to placebo condition (t = 3.00; p < 0.04; Fig. 5).

Sleep characteristics. Nocturnal sleep.
Given that the duration of wakefulness was experimentally prolonged prior to the initiation of nocturnal sleep, in both conditions, participants showed a sleep onset latency shorter than 10 min and more than 2 h of deep stage N3 sleep. For all sleep variables analyzed (Table 1), the linear mixed-effects models revealed no significant main effect of "condition" (F = 0.56; p > 0.05; η 2 = 0.0025). Nevertheless, post hoc testing with the 'R' package emmeans indicated that the time spent in N3 sleep was shorter (p = 0.004) in the caffeine condition when compared to the placebo condition ( Fig. 6A; Table 1).
Nap sleep opportunity. The statistical analyses of the sleep variables in the nap opportunity 1 h after scheduled awakening revealed a significant 'condition' effect (F = 2.09; p < 0.01; η 2 = 0.87), such that sleep onset latency and wakefulness after sleep onset were prolonged. In addition, the time spent in stage N2 was reduced in the caffeine condition when compared to the placebo condition (p all < 0.01; Fig. 6C; Table 1).

Discussion
Here we tested the in vivo drug release profile and efficacy to ameliorate morning sleep inertia of a delayed pulsatile-release caffeine formulation administered at bedtime. We found that this innovative approach potently facilitated the sleep-to-wake transition on neurobehavioral, subjective and physiological markers of sleep inertia The in vivo validation study corroborated the intended release profile. As expected based on the in vitro development of the engineered caffeine micropellets, the caffeine curve followed a delayed-release profile, and an efficacious plasma concentration above ~ 5 μM was attained only after 7 h. Unexpectedly, the release profile of the identical formulation in the pharmacodynamic study was distinctly different from the validation study. Premature caffeine-release started already 3.5 h after drug administration and efficacious plasma levels were attained already 5 h after drug intake. We suggest that a premature burst of the formula was probably triggered by the meal served to the subjects 3.5 h after caffeine administration (this meal was not served in the validation   www.nature.com/scientificreports/ study). This food intake may have promoted gastric movements that caused faster gastric emptying and provoked the break of the release-modifying polymeric coat due to the physical impact of the peristalsis. Further studies are needed to confirm this hypothesis. In addition, future research may also clarify whether the pharmacokinetics of the engineered caffeine formula systematically differs between sleep and wakefulness. Despite the premature release of caffeine, the formula ameliorated sleep inertia on the subsequent morning following only 4 h of sleep. The quality of awakening was subjectively improved on behavioral, cognitive, emotional, and physical levels, as indicated on all subscales of the sleep inertia questionnaire. Even though the study participants were sleep restricted, many of them reported less difficulty to rise when compared to a habitual morning, particularly in the caffeine condition. The formulation also exhibited mood enhancing properties, as indicated by increased positive and reduced negative ratings on the PANAS. This finding may suggest increased activity of mood-relevant brain structures in the caffeine condition when compared to the placebo condition. Caffeine blocks adenosine A 2A receptors in the nucleus accumbens 50,51 , a core region of the mesolimbic dopamine network that is essential for the generation of positive mood and reward 52 . We speculate that caffeine-induced dopamine release in the nucleus accumbens 53 could contribute to the post-awakening mood enhancement when compared to placebo. Other possible mechanisms include increased preparatory attention for rewarding stimuli 54 or direct interaction with sleep-wake regulatory pathways such as the circadian clock 55 . The mechanistic underpinnings of the benefit of the pulsatile caffeine-release formulation on subjective state will have to be clarified in future studies.
The arousal effect of caffeine relies on the competitive antagonism of central nervous system adenosine receptors that contribute to the regulation of sleep intensity and sleep need 14,50 . Sleep inertia, in particular upon sleep restriction, was previously proposed to reflect 'adenosine left overs' that were insufficiently removed during sleep 1,56 . Consistent with this view, the primary beneficial effects of the tested caffeine release formula were not restricted to subjective state but included improved vigilance as manifested by faster PVT reaction times when compared to placebo. This finding is important because after nocturnal sleep, no easy applicable proactive or efficacious reactive countermeasure to impaired neurobehavioral performance due to sleep inertia is currently www.nature.com/scientificreports/ available 6 . Previous work suggested reduced lapsing on the PVT immediately after waking from a 30-min coffee-nap ending at 04:00 24 and from repeated 2-h sleep opportunities during prolonged sleep deprivation 16 . In the present study, we observed no changes in the number of PVT lapses. The differences in the experimental protocols, as well as the caffeine dosages and application forms may underlie the discrepancy. Furthermore, we found no improvement in cognitively more demanding tasks such as sustained selective attention and visual scanning speed (assessed with the d2-task) and working memory and executive functioning (n-back task). These tasks were administered > 45 min after waking. Rested baseline measurements would be necessary to determine whether performance on these tasks at the time of their administration was impaired by sleep inertia and could be improved with the intervention tested. After caffeine, the cortisol level on the CAR was increased 1 h after wake-up when compared to placebo. This finding supports the hypothesis that caffeine stimulates cortisol secretion and augments the CAR upon waking. The wake-up-related cortisol secretion was previously suggested to reflect a hormonal wake-promoting signal 20 . Nevertheless, neither the peak cortisol concentration nor the area-under-the-curve were affected by caffeine, suggesting that a direct association between reduced sleep inertia within the first 15 min of waking and HPA-axis activity is rather unlikely. Recent studies in rats revealed that overexpression of adenosine A 2A receptors may contribute to glucocorticoid receptor dysfunctions in aged animals and that caffeine may re-sensitize glucocorticoid receptors in the hypothalamus and restore HPA-axis function 57 . Although the observed effect on the CAR in our young healthy sample was subtle, it may be speculated that a delayed-release caffeine formulation may promote cortisol release and glucocorticoid receptor functioning in susceptible individuals 58 .
Due to the sustained-release profile of the engineered formula, the blood caffeine concentration remained within an efficacious dose range until 17.5 h after administration. Consistent with this pharmacokinetic profile, the increased wake time after sleep onset, the prolonged sleep latency and the reduced N2 sleep duration during the 1-h nap after awakening support the notion that sustained low-dose caffeine administration improved post-awakening vigilance 16 . The premature high concentration of caffeine during nocturnal sleep most likely also underlies the ~ 17-min reduction in deep N3 sleep observed during the main sleep episode. Such a reduction in deep sleep would hamper the applicability of this novel pulsatile-release caffeine formula. Ongoing research employing quantitative sleep EEG analyses as a function of caffeine levels during sleep, as well as follow-up sleep studies will determine whether sleep is also disturbed without intra-night food intake. With respect to the reduced sleep depth, it seems unlikely that the mitigated sleep inertia after waking depended on the reduced duration of N3 sleep because this sleep state did not differ between the conditions during the final 10 min before scheduled awakening.
Taken together, this proof-of-concept investigation demonstrates that a timed pulsatile-release caffeine system ingested at bedtime can potently attenuate neurobehavioral, subjective, emotional and physiological manifestations of morning sleep inertia in sleep-restricted healthy young men. These findings cannot be generalized because only a single dose, only healthy men, and individuals irrespective of their caffeine sensitivity were studied. Nevertheless, if future research supports these conclusions and further improves the drug-release profile of the engineered formula, time-controlled caffeine administration may be developed as an add-on therapy to mitigate impaired morning state and vigilance in people suffering from excessive sleep inertia, which is highly prevalent in on-call and shift work settings, as well as in patients with neurological, neuropsychiatric, and circadian-rhythm sleep-wake disorders.